What is an Alternate Key in DBMS?

If you are working with databases, it is important for you to have a solid understanding of how data is organized and accessed in an efficient way. One of the important concepts in databases includes the use of DBMS keys. These keys help you ensure that the records are identified in a unique way and can be retrieved quickly. Among the various types of keys, the one key that often raises questions is the Alternate Key.

In this blog, we are going to walk you through everything you need to know about alternate keys in DBMS. So let’s get started!

Table of Contents:

• What Are Keys in DBMS and Why Are They Important?
• What is an Alternate Key?
• Characteristics of an Alternate Key
  1. Uniqueness
  2. Not-Null Constraint
  3. Candidate Key
• SQL Syntax & How to Create an Alternate Key
• Alternate Key Example in SQL Table with Output
• Why Alternate Keys Matter in DBMS: Benefits & Use Cases
  1. Data Retrieval Flexibility
  2. Role of Alternate Keys in Database Normalization
  3. Adapting to Business Logic
• Primary Key vs Alternate Key: Key Differences
• Alternate Key vs Unique Key
• SQL Constraints Used to Create Alternate Keys
• Practical Applications and Real-World Use Cases of Alternate Keys
• Conclusion

What Are Keys in DBMS and Why Are They Important?

In DBMS (Database Management System), a key is considered to be a specified field or a combination of fields that are used to identify each record in a table in a unique way. DBMS Keys are useful for maintaining data integrity and enabling efficient data retrieval. Without the use of DBMS keys, finding and referencing specific records would be slow and prone to errors. Among the most commonly used keys, the Primary Key is useful for identifying each row in a unique way. The Candidate Key works as a field that can serve as a unique identifier; the Alternate Key is a Candidate Key that is not chosen as the Primary Key; and the Foreign Key is responsible for linking records between different tables. Together, these SQL keys build the foundation for organizing and managing relational data effectively.

What is an Alternate Key?

An alternate key is any key in a database that is responsible for identifying a row in a unique way, but is not chosen as the primary key. When there are multiple candidate keys in a table (all capable of identifying rows in a unique way), only one key is selected as the primary key, and the rest of them are considered as alternate keys. These alternate keys act as additional unique identifiers. They allow you to access data in the table in other ways, especially if it is not ideal or possible to use the primary key. Having alternate keys in a table adds flexibility and helps you to ensure that there are multiple reliable options for identifying records in a unique way.

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Characteristics of an Alternate Key

Some of the important characteristics of an Alternate Key are given below:

1. Uniqueness

Just like a primary key, an alternate key should have unique values. This means that no two rows in the table can have the same value for the alternate key. It is important to have this uniqueness because it helps you to ensure that the alternate key can be used to identify and distinguish each record in the table in a correct way. This is a fundamental property of Alternate Key constraints in SQL.

2. Not-Null Constraint

An alternate key has the potential to become a primary key. Therefore, it must follow similar rules, one of them being that it cannot contain null values. It is compulsory for each record in the table to have a value in the alternate column. This is important because if there is a missing value (null), that record cannot be identified uniquely. This leads to problems with data accuracy and integrity in the database.

3. Candidate Key

A candidate key is any field that can uniquely identify a row in a table. An alternate key is one of those candidate keys that was not chosen as the primary key but has the qualities to serve as one. If there are issues with the current primary key and it no longer meets certain requirements, you can use an alternate key instead. The choice depends on the database design and what best fits the situation.

How to Create Alternate Keys in SQL with UNIQUE Constraint

An alternate key is one of the SQL Keys that is not defined using a special keyword. It is created by applying the UNIQUE constraint to a column or a set of columns that contain unique values, but is not the primary key. This helps you to ensure that no duplicate values can be entered in that column, thus allowing it to serve as a backup unique identifier. Also, the term “alternate key” is not used directly in SQL; any column with the UNIQUE constraint, aside from the primary key, acts as the alternate key.

Here is an example given below for your reference.

Example:

-- Step 1: Create the table
CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    phone VARCHAR(15) UNIQUE,
    name VARCHAR(50)
);

-- Step 2: Insert sample data
INSERT INTO Employees (emp_id, email, phone, name) VALUES
(1, '[email protected]', '1234567890', 'emp1'),
(2, '[email protected]', '2345678901', 'emp2'),
(3, '[email protected]', '3456789012', 'emp3');

-- Step 3: Select from the table to generate output
SELECT * FROM Employees;

Output:

Explanation:

In the above table, emp_id is the primary key, while email and phone are alternate keys. This is because they are unique as well and can identify each employee individually.

Alternate Key Example in SQL Table with Output

An alternate key is basically a column (or combination of columns) that is used to identify a record in a table in a unique way. They are not selected as the primary key. It serves as a backup unique identifier. For example, in an employee table, both email and employee_id might be unique, but if you choose employee_id as the primary key, then email becomes the alternate key.

An example is given below for your reference.

Example:

-- Step 1: Create the table
CREATE TABLE Employees (
    employee_id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    name VARCHAR(50)
);

-- Step 2: Insert sample data
INSERT INTO Employees (employee_id, email, name) VALUES
(101, '[email protected]', 'emp1'),
(102, '[email protected]', 'emp2'),
(103, '[email protected]', 'emp3');
-- Step 3: Query the table to generate output

SELECT * FROM Employees;

Output:

Explanation:

In the above query, employee_id is the primary key, and email is the alternate key. This is because it is also unique, but not the main identifier.

Why Alternate Keys Matter in DBMS: Benefits & Use Cases

Given below are the key reasons why alternate keys are important.

1. Data Retrieval Flexibility

Alternate keys provide you with alternate ways to find data in a table when the primary key is not available for a specific query. This can be helpful because you might need to search for records using various fields, and by having alternate keys, you can still get accurate results.

2. Role of Alternate Keys in Database Normalization

In database normalization, where it helps you to reduce repeated data and avoid dependency issues, alternate keys can be useful. They help you to improve the structure and reliability of the database by giving extra ways so that you can access and connect to data without causing errors or duplication.

3. Adapting to Business Logic

In real-life business scenarios, you need to identify records by using something other than the primary key. Alternate keys help you to do this by providing you with more options, which can help you to identify data in a unique way, based on the business needs or how the data is used.

Primary Key vs Alternate Key: Key Differences

Here is a comparison table that shows the difference between primary key and alternate key in DBMS based on different aspects:

Alternate Key vs Unique Key

SQL Constraints Used to Create Alternate Keys

In SQL, alternate keys are created using the UNIQUE constraint. There is no special ALTERNATE KEY keyword in SQL. Instead, any column or combination of columns defined with UNIQUE, but not as the PRIMARY KEY, acts as an alternate key, because it ensures the values remain unique and non-NULL.

Here are the examples that highlight the practical application of alternate key constraints in SQL.

Example 1: Single-column alternate key

CREATE TABLE Employees (
    emp_id INT PRIMARY KEY,        -- primary key
    email VARCHAR(100) UNIQUE,     -- alternate key: unique constraint
    name VARCHAR(50)
);

Explanation: This SQL statement creates an Employees table where emp_id is the primary key and email has a UNIQUE constraint, making it an alternate key. The name column stores employee names without any unique constraint.

Example 2: Multi-column alternate key

CREATE TABLE Orders (
    order_id INT PRIMARY KEY,
    product_id INT,
    customer_id INT,
    -- alternate key on (product_id, customer_id)
    UNIQUE (product_id, customer_id)
);

Explanation: This SQL statement creates an Orders table with order_id as the primary key. The combination of product_id and customer_id has a UNIQUE constraint, which forms an alternate key that ensures each product-customer pair is unique across orders.

Example 3: Named UNIQUE constraint

CREATE TABLE Customers (
    customer_id INT PRIMARY KEY,
    national_id VARCHAR(20),
    CONSTRAINT unique_national_id UNIQUE (national_id)  -- named alternate key
);

Explanation: This SQL statement creates a Customers table with customer_id as the primary key. It defines a named UNIQUE constraint (unique_national_id) on national_id, making it an alternate key so each customer’s national ID must be unique.

Practical Applications and Real-World Use Cases of Alternate Keys

Given below are some common scenarios where you can find the alternate keys to be valuable.

• User Authentication: A user might log in using his/her email or phone number. So both the email and the phone number must be unique to make sure that the system can identify the user correctly.

• Data Validation: Alternate keys help to prevent duplicate entries in fields like national ID numbers or tax identification numbers.

• Query Optimization: Alternate keys allow you to search or join tables using unique, indexing fields other than the primary key.

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Conclusion

Having a good understanding of SQL keys will help you design better and reliable databases. An alternate key works as a unique identifier, just like the primary key, but it is not chosen as the main identifier. It helps to strengthen data integrity, enhances the flexibility of querying, and makes the database more versatile.

By properly using alternate keys, you can ensure that your database is not only efficient but also resilient to issues regarding duplication of data. If you are building a relational database, you must always consider the fields that might serve as alternate keys; otherwise, you may face problems in the future. Thus, understanding DBMS keys and their variations, including Alternate Key constraints in SQL, is important for robust database design.

Learn what a secondary key is in DBMS and how it is used in this blog.

More Related Topics

• Normalization in SQL: 1NF, 2NF, 3NF, and BCNF in DBMS
• Super Key in DBMS
• SQL Constraints 
• DBMS Interview Questions

What is an Alternate Key in DBMS – FAQs